In recent years, various power systems using a commercial power source and a storage battery are widely used. In such a power system, the commercial power source and the storage battery are connected to each other via a power conversion device or the like that bi-directionally converts power between DC power and AC power. The commercial power source and the storage battery exchange power by use of the power conversion device. Many of the power conversion devices are designed to obtain the maximum power conversion efficiency when operating at a value close to the rated value. However, the operating state of the power conversion device may fluctuate due to various factors, such as consumed power of a load connected thereto. Therefore, the power conversion device cannot always operate at a value close to the rated value. In other words, the power conversion device may possibly operate at a value considerably smaller than the rated value, which could cause the risk of increasing the loss because the power conversion efficiency of the power conversion device considerably lowers.
Japanese published application 1 discloses an example of a technology of reducing the loss at the time of power conversion by power conversion devices. An AC power supply device disclosed in Japanese published application 1 has a plurality of DC-AC inverters.
The AC power supply device selects the combination of the plurality of DC-AC inverters to maximize the power conversion efficiency of each of the DC-AC power inverter based on the relationship between the output power and the conversion efficiency of each of the DC-AC inverters. Thereby the AC power supply device can reduce the loss of each of the DC-AC inverters at the time of power conversion.